Emission standards for sulfur dioxide and nitrogen oxides were established with Title IV of the Clean Air Act Amendments of 1990. To meet these standards, continuous emission monitoring (CEM) systems were required to monitor these gases as well as carbon dioxide or oxygen, diluent gases, flue gas velocity, and opacity.
For all CEM systems, the measured results may be biased, i.e., inaccurate, due to a number of factors. Some of these factors relate to limitations or inherent problems in the equipment used in the system, while other factors are caused by changes in the stack which therefore result in changes in the extracted sample that are not compensated for by the system's components. Many of the factors contributing to bias for various types of CEM systems are discussed in "An Operator's Guide to Eliminating Bias in CEM Systems", United States Environmental Protection Agency, EPA 430-R-94-016.
A typical dilution extractive CEM system comprises a dilution probe within the stack, a gas conditioning system, a gas analyzer located proximate the stack, and a remote computer. Generally, stack (emission) gas is extracted from the stack through a sonic orifice in the dilution probe. Then clean gas (dilution air) is injected into the probe and mixed with the sample emission gas to produce a diluted sample which is then analyzed by the analyzer. Further analysis of the sample is achieved at the remote computer.
Extractive CEM systems using a dilution probe are often employed when a need exists to filter particulate matter that may be in the stack. Generally, when dilution probes are used, only the gas from the source enters the probe and the particulate matter does not enter the probe. Further, such systems draw gas from the stack at a rate significantly less than the source.
Dilution extractive CEM systems are generally application dependent, but yet had been proven to meet various applicable standards, such as linearity, calibration drift, and accuracy. Therefore, these CEM systems gained wide recognition as the system of choice. Recently, however, Environmental Protection Agency (EPA) standards have been promulgated which are more stringent than those of 1990. These new regulations caused problems with CEM systems in meeting the required accuracy.
To remain within the EPA requirements, many current CEM systems must generally be calibrated daily or every few days. Frequent calibration is not only costly, but it causes the user to lose valuable data, as data cannot be collected during the calibration process. Therefore, it is desirable to develop a dilution extractive CEM system which meets the new EPA requirements and does not require that the system be frequently calibrated.
One of the components having the largest impact on the accuracy of a dilution extractive CEM system is the dilution probe, such as the EPM Probe Heater Assembly Models 797.560/561 manufactured by EPM B. V. of Dalerstraat, Netherlands, and distributed in the United States by EPM Environment, Inc. of Mt. Prospect, Ill. The critical flow orifice extracts the gas at sonic velocity. The sonic velocity of the gas varies directly proportional to the square root of the density of the gas being analyzed. Generally, by calculating the change in density, it can be determined how much the dilution ratio has changed, and the appropriate adjustments can be made to achieve a correct reading.
In general, the problems associated with the dilution probe that result in bias are changes in: (1) absolute stack pressure; (2) stack temperature; (3) gas density; and (4) water droplet evaporation. Absolute stack pressure is the sum of the barometric pressure and the stack static pressure. Therefore, changes in absolute stack pressure result from changes in plant operating condition or from changes in weather conditions (barometric pressure). With regard to temperature, cessation of operation of the source will result in a large temperature change. The effects of temperature and pressure was studied by the Electric Power Research Institute. "Pressure and Temperature Effects in Dilution Extractive Continuous Emission Monitory Systems", Electric Research Institute, EPRI TR-104700, December, 1994.
To compensate for the non-linear changes resulting from changes in temperature, a heater is often placed around the dilution probe to maintain the probe at a constant temperature. Even if such corrective action is taken, corrections are still needed for the linear effect of change in absolute pressure. Generally, changes in pressure have been accommodated by correcting the collecting data according to the known linear relationship with the remote data acquisition system.
It is possible for corrections to be made for the effects of gas pressure (absolute pressure) and temperature on the gas density and gas analysis in a dilution extractive CEM system by calculating the effects and making appropriate adjustments. As previously stated, one way these adjustments are currently made is to correct the measured value after the analyzer has made its reading, i.e., to correct the value with the remote data acquisition system.
There are several problems associated with correcting the value at a remote computer. Latent correction means that any display of data collected near the probe is incorrect--the correct values can only be seen at the remote computer, not at the analyzer. Because calibration of the system occurs away from the remote computer, a display must be located near the analyzer, and that display must necessarily be connected to the remote computer. It is therefore desirable to provide a CEM system which corrects for changes in gas density, including changes in absolute stack pressure and stack temperature, without requiting that the corrections be made a remote computer sight.
It is also desirable to provide a system for correcting for changes in gas density that may be used with different types of analyzers as different detection methods may be used. Such universality of application permits use of the corrections on existing CEM systems as well as future CEM systems.
One of the advantages of correcting the collected data with the data acquisition system is the fact that the capability to correct the data may easily be added into an existing system. The "retrofitability" stems from the fact that computer software is added to the already existing data acquisition system to correct for changes gas density. Therefore, it is desirable for any other method and apparatus provided to correct for changes in gas density to also be able to be retrofitted into existing systems. Such a retrofit should be cost-conscious, not only as to the purchase and maintenance costs of any additional or modified equipment that may be necessary, but also with regard to the costs associated with installation of the retrofit.